1. Field of the Invention
The invention relates to a semiconductor laser element and, in particular, to a semiconductor laser element having a semiconductor layer including an active layer formed a GaN layer.
2. Description of the Related Art
In the field of manufacturing a high-density optical disc memory or printing using a photosensitive material, a semiconductor laser of 400 nm band having a micro-spot is expected to oscillate a fundamental transverse mode by a highly reliable Gaussian beam.
Disclosed in Jpn. J. Appl. Phys., Vol. 37, pp. L1020, issued in 1998 is xe2x80x9cInGaN/GaN/AlGaN-Based Laser Diodes Grown GaN Substrates with a Fundamental Transverse Modexe2x80x9d by Nakamura et al. This is formed as follows: a GaN layer is formed on a sapphire substrate and then the thick GaN film formed by the use of selective growth by using SiO2 as a mask is removed to make a GaN substrate, and on the GaN substrate are formed an n-GaN buffer layer, an n-InGaN crack preventing layer, an n-AlGaN/GaN modulation doped superlattice clad layer, an n-GaN optical waveguide layer, an n-InGaN/InGaN multiple quantum well active layer, a p-AlGaN carrier block layer, a p-GaN optical waveguide layer, a p-AlGaN/GaN modulation doped superlattice clad layer, and a p-GaN contact layer. However, in this case, a refractive index guiding type structure is formed by forming a ridge structure of about 2 xcexcm and this semiconductor laser can produce only the fundamental transverse mode oscillation, at most, of about 30 mW because it is very difficult to control the depth of etching. Further, although a try to reduce an element resistance is made by the use of the modulation doped superlattice clad layer, the element resistance is not sufficiently reduced and hence the reliability of the element is observed to be degraded by the joule heat generated when the element is operated.
As described above, the above-mentioned structure presents a problem that a single mode laser having a small contact area with a contact layer is affected in a practical use by heat generation because the element resistance is large.
The invention has been made in view of the above problem, and it is an object of the invention to provide a semiconductor laser element producing a Gaussian beam of high quality having high reliability to a high power by reducing an element resistance and preventing the effect of heat generation.
A first semiconductor laser element in accordance with the invention is composed of a GaN layer and at least a lower clad layer, a lower optical waveguide layer, an active layer, an upper optical waveguide layer, an upper clad layer, and a GaN contact layer all of which are laminated on the GaN layer in this order, is provided with a current injection window above the active layer, and is further composed of a first AlGaN composition gradient layer which is formed between the GaN layer and the lower clad layer so that a band gap thereof continuously changes from the GaN layer to the lower clad layer and a second AlGaN composition gradient layer which is formed between the upper clad layer and the GaN contact layer so that a band gap thereof continuously changes from the upper clad layer to the GaN contact layer.
Further, the first semiconductor laser element in accordance with the invention may be composed of a third AlGaN composition gradient layer which is formed between the lower clad layer and the lower optical waveguide layer so that a band gap thereof continuously changes from the lower clad layer to the lower optical waveguide layer and a fourth AlGaN composition gradient layer which is formed between the upper optical waveguide layer and the upper clad layer and so that a band gap thereof continuously changes from the upper optical waveguide layer to the upper clad layer.
It is desirable that the above-mentioned first semiconductor laser element in accordance with the invention has an equivalent refractive index difference which is not less than 0.002 and not more than 0.01 in the case where a stripe width is not less than 1 xcexcm and not more than 2.5 xcexcm and that it has an equivalent refractive index difference which is not less than 0.002 and not more than 0.015 in the case where a stripe width is not less than 2.5 xcexcm.
The second semiconductor laser element in accordance with the invention is composed of a GaN layer and at least a lower clad layer, a lower optical waveguide layer, an active layer, an upper optical waveguide layer, an upper clad layer, and a GaN contact layer all of which are laminated on the GaN layer in this order, is provided with a current injection window having a predetermined stripe width above the active layer, and is further composed of the first AlGaN/GaN quantum well gradient layer which is formed between the GaN layer and the lower clad layer and which has an energy gap larger than the GaN layer and smaller than the lower clad layer and/or the second AlGaN/GaN quantum well gradient layer which is formed between the upper clad layer and the GaN contact layer and which has an energy gap larger than the upper clad layer and smaller than the GaN contact layer.
Here the term, xe2x80x9cthe energy gapxe2x80x9d refers to a substantial energy gap, and the term xe2x80x9cthe energy gap in the quantum well gradient layerxe2x80x9d refers to a substantial energy gap obtained in consideration of a tunnel effect.
That is, in the second semiconductor laser element in accordance with the invention, a difference in the energy gap between the neighboring layers is reduced by providing the AlGaN/GaN quantum well gradient layer having an energy gap of intermediate magnitude between the energy gaps of the upper and lower layers, the upper and lower layers being the GaN layer and the lower clad layer and/or the contact layer and the upper clad layer.
Here, the quantum well gradient layer is composed of at least one quantum well layer and a pair of barrier layers sandwiching the quantum well layer. The quantum well gradient layer may be a single quantum well gradient layer having an energy gap connected stepwise to the energy gaps of the upper and lower layers, or may include multiple quantum well gradient layers. Also, in the case of the multiple quantum well gradient layers, the respective multiple layers may be constituted so that the energy gaps of the respective multiple layers continuously change and connect the energy gap between the upper and lower layers.
It is desirable that the second semiconductor laser element in accordance with the invention is further composed of the third AlGaN/GaN quantum well gradient layer which is formed between the lower clad layer and the lower optical waveguide layer and which has a band gap smaller than the lower clad layer and larger than the optical waveguide layer, and/or the fourth AlGaN/GaN quantum well gradient layer which is formed between the upper optical waveguide layer and the upper clad layer and which has a band gap smaller than the upper clad layer and larger than the upper optical waveguide layer.
Also in this case, as described above, the respective quantum well gradient layers may be a single quantum well gradient layer or may include multiple quantum well gradient layers. Also, in the case of the multiple quantum well gradient layers, the respective multiple layers may be constituted so that the energy gaps of the respective multiple layers change in a stepwise or continuous manner.
Of the barrier layers and the quantum layers constituting the respective quantum well gradient layers, only the barrier layers may be doped with impurities or both of the barrier layers and the quantum layers may be doped with impurities.
It is desirable in the second semiconductor laser element in accordance with the invention that the above-mentioned stripe width is not less than 1 xcexcm and not more than 2.5 xcexcm and that an equivalent refractive index difference is not less than 0.002 and not more than 0.01 or that the above-mentioned stripe width is not less than 2.5 xcexcm and that an equivalent refractive index difference is not less than 0.002 and not more than 0.015.
Since the first semiconductor laser element in accordance with the invention includes the first AlGaN composition gradient layer which is formed between the GaN layer and the lower clad layer so that the band gap thereof continuously changes from the GaN layer to the lower clad layer and the second AlGaN composition gradient layer which is formed between the upper clad layer and the GaN contact layer so that the band gap thereof continuously changes from the upper clad layer to the GaN contact layer, the height of the barrier can be reduced because of the band offset produced between two layers sandwiching the composition gradient layer. This reduces the element resistance preventing a deterioration in characteristic caused by heat generation. Therefore, the characteristic and reliability of the semiconductor laser element can be improved providing a beam of high quality.
Further, since the first semiconductor laser element in accordance with the invention may include the third AlGaN/GaN composition gradient layer which is formed between the lower clad layer and the lower optical waveguide layer and which has a continuously changing band gap and/or the fourth AlGaN/GaN composition gradient layer which is formed between the upper optical waveguide layer and the upper clad layer and which has a continuously changing band gap, the height of the barrier can be reduced in the same way as described above because of the band offset produced between two layers sandwiching the composition gradient layer. This reduces the element resistance. Therefore, the characteristic and reliability of the semiconductor laser element can be improved providing a beam of high quality.
Since the first semiconductor laser element in accordance with the present invention has a stripe width of not less than 1 xcexcm and not more than 2.5 xcexcm and an equivalent refractive index difference of not less than 0.002 and not more than 0.01, it can produce a fundamental transverse mode oscillation of high quality. Also, since the first semiconductor laser element in accordance with the invention has a stripe width of not less than 2.5 xcexcm and an equivalent refractive index difference of not less than 0.002 and not more than 0.015, it can produce a stable oscillation mode even if multiple modes are included.
Since the second semiconductor laser element in accordance with the invention includes the first AlGaN/GaN quantum well gradient layer which is formed between the GaN layer and the lower clad layer and which has an energy gap larger than the GaN layer and smaller than the lower clad layer and/or the second AlGaN/GaN quantum well gradient layer which is formed between the upper clad layer and the GaN contact layer and which has an energy gap larger than the GaN contact layer and smaller than the upper clad layer, the height of the barrier can be reduced because of the band offset produced between two layers sandwiching the quantum well gradient layer. This reduces the element resistance as a whole preventing a deterioration in characteristic caused by heat generation. Therefore, the characteristic and reliability of the semiconductor laser element can be improved providing a beam of high quality.
Further, the second semiconductor laser element in accordance with the invention may include the third AlGaN/GaN quantum well gradient layer which is formed between the lower clad layer and the lower optical waveguide layer and which has a band gap smaller than the lower clad layer and larger than the optical waveguide layer, and/or the fourth AlGaN/GaN quantum well gradient layer which is formed between the upper optical waveguide layer and the upper clad layer and which has a band gap smaller than the upper clad layer and larger than the upper optical waveguide layer, the height of the barrier can be reduced in the same way as described above because of the band offset produced between two layers sandwiching the GaN contact layer gradient layer. This reduces the element resistance. Therefore, the characteristic and reliability of the semiconductor laser element can be improved providing a beam of high quality.
Still further, since the second semiconductor laser element in accordance with the invention has a stripe width of not less than 1 xcexcm and not more than 2.5 xcexcm and an equivalent refractive index difference of not less than 0.002 and not more than 0.01, it can produce a fundamental transverse mode oscillation of high quality.
Still further, since the second semiconductor laser element in accordance with the invention has a stripe width of not less than 2.5 xcexcm and an equivalent refractive index difference of not less than 0.002 and not more than 0.015, it can produce multiple modes being as a whole stable oscillation mode with little noises.